YA SciFi/ Fantasy? Boy who can go from one dimension to another and fight with the good demons/ monsters. Title includes "Key"? Read around 2015.

[Alyx]

I read a book about 7 years ago and for some reason i feel like it has key in the title maybe? Its about a boy who can see/go into a different dimension with demons/monsters who need his help in order to defeat a very bad demon/monster. Im pretty sure he had like one or two friends who could do the sam thing as him, and he had to help the demons to fight the really evil demon. They fight one and end up killing him but then his sister wants vengeance and kidnaps one of the good demons, so the boy has to like help save the kidnapped one. And then in the end I think he has to decide whether or not he stays in the different demention or real life

[Rainbowheart]

The Marbury Lens?